Manufacture of chlorine dioxide



Nov. 4, 1952 H. c. MARKS ET AL 2,616,792

MANUFACTURE OF' CHLORINE DIOXIDE Filed April l. 1949 Patentecl Nov. 4, 1952 UNITIELD;v STATES. PATENT oFF-ics 2,616,792 MANUFACTURE oF-f'CnLoRINE- DIoXiDEI Henry-e0. .MarkS,....Glen` Ridge. and Robert R.

Jgner, Newark, N. -T J., assignorsto Novadele..r Agene orporation, Belleville, N.- .La corporate.A

tion of Delaware AlpplcationJApril 1', v19419, ISerial-No; 85;!) 62 8, .Claims. (Cl. 23e-152),.

Thisfinventionrelatesftomethodsofiproducing-r chlorine dioxide;` .which is .tamenergetic oxidizingv f agent having important?..-.imlustrial; 'and other;`

uses. Under the:v conditions;1at; .Wh-ich .it` ispzcusf tomarily employed chlorine;` dioxide .is a` gas.ar.1d.. moreover has awtendencywto;.-decompose;usuallyf with explosive. violence, .unless handled .withconf siderable l care. Fori convenience; and; safety, a

preferred practice .isto sproducev chlorine.I dioxide it only: as requiredl,..1so.v.fthat..there is no .needrtO store theigas v:for/any :length/.of time. or: to paclrage it for transportation Asaaffurther. safeguard;

against explosion .-and;against.:;1other: harm that might result'fromf.inadvertentfreleasegof thef'pure, reagent;` it= is ordinarilwdilutedxfto.arlarge: extent with otherV gaszWhichfis-'suitablyinertqto chlorine dioxide'. and .also2iner.t', soifari'as Incessary,` ;to.; all

materials that. mustf be :exposed2 to. the diluted .e

mixture.`

For most purposes;.-, assatisfactoryfev method of easily contro1led,.without-1 expert.;attention,- and. thatl uses commercially. ;.available;;materials; and can `be carried'outwithisimpleslnexpensiveequin: 1 ment; Ai particularlyi importantfactor is. that of safety;` with respect tore-:explosion:andyother haZg-,r ards, -f since the. gas.L7 oftena'hasfto `loer; produced by more-orVLV less autornai-ic'zi'equipmentwhichris;y left to l the attention4` v(at i leastulffor:considerable.v p e-l riods) l of! persons .l havingf: noi-special ;technic al, understandingof.-` apparatus and aprocedurevin. the,

art of manufacturing. thisgas; orfinhandlingpd- -tentiallydangerousreagents Accordingly it is tially-foolproofe and' requiressfno'f. such' reagents.. For example, an elsewl'ie'erefproposedimethodafor.` making chlorine dioxidef-byilrstfadding. concentratedV nitricacid Yto atchlorat'e solutionftofprovide;

a concentrated solution-V ofi Enitricif` and.` chloric tercurrently- (i: e. in t opposites. directions. through `-a reaction `tower-ff 1i ilike.: unsuitable forf'purpose's alcove,v sinceY it involves with -due--care` the primary ingredient ff'concengf trated nitric acidfisolution but ;theffstrong. solu-` tion of nitric and chlorictacids.ideliveredetosthe..

reaction.l tower lis a=1-veryi hazardous fmaterial, for

example 1in-1 that. anyficontactawitli only: a small amountJofforganiczmatteri canzi1resultsinga, Violent arelativelyf': ortfmentioned. arelativelyl complex; .f and.. hazardousl process.- Notfliionly: mustmfthereubec equipment andi vpersonnel'fcapahlei notl fhandling.:

- acids, kland then :running:4 the: produced l reagent solution `and "diluted nitrogen: peroxideigas .conn-1 Irl-addition to thedesifamutypoth ofavoidmg the handling 0f dangerous substances, andl of af fording a relatively inexpensive .n1eihod,a in,. out n standing requirement is,. ease of control of the, process relativev to the output ofchlorinefdioxide.., Where large but variable quantities-f of agCQmr. modity ormaterial 4aretdloe treated with the dilute gaseousv product, for'fexample A,as gin; the -f flour industry, both the economiefactorand the@ facility ofgcontrol are ;:especiallyl-to leeg-desired.v Since convenient practicein makingichlorine difV oxide for these .vpurposes. ;is;,;to prox/ ideacontinnous or. relatively continuousfftype `ofoperation Wherev reagent materialsnarafed constantly; toga; reaction zone from `whichthe'f desired and other.;A products. are .withdrawn lin.: a; continuous man: f ner, it is extremely advantageousytoibe,ahlejiocontrol 4theratesoilproduction.11cm adjustilrigfl the rate. of feed' of the; lreagents., For.; that ,purpose the proportionality, of efiluent chlorine dioxideetOe. input fmaterialsshould .be relatively-uniform :over: a considerable. rangeof. ratesaof operation. Inzi deed it is espeiallydesirable thatsuchfprOROl-v tionality; he; maintainedybetween the inputgof; a i single reagent, such Yaspa reagent gas; and the output of chlorine dioxide, evenggthpllgh Othern; supplied reagent .materia1. mayrnct ;b.e fvaredain;

rate offeedeXactly-asthe reagent gas. .g

suits, with -use ful .economy of reagent', both gaseous andv non-gaseous, so thatfor'. example; in preferred typesoff operation at .least about l of each y incomingk reagent l mate rial,- is., usefully l consumed for the pr-oductionpf chlorine; dir )xide1 lt wilLnoW .be seenlthat under the describedcrcumstances of proportiona1ity, asl attained 4Icy the. present invention, the-flowof treating.gas,,.viz. diluted l chlorine dioxide, to .the locality of use-v canlpe easily and` accurately controlledwith-,re-

spect y.to its-g` active y ingredient, l simply by controlling (i. Ve...adjus ti1f1g .A the supplyof reagent gas, or at least by conjoint regulationoftfeed of,both

Infan. aspect. or chief imncrtancexthe present?.-

invention embraces the discovery that the foregoing criteria of reliability, safety, economy and ease of control may be achieved by a procedure involving the passage of diluted nitrogen peroxide gas and a solution of a soluble chlorate such as sodium chlorate or calcium chlorate through a reaction zone, wherein certain conditions of proportionality, concentration and physical situation are satisfied within certain limits that have now been discovered and that are in large measure critical for the useful realization of chlorine dioxide production by reaction between nitrogen peroxide and such a chlorate solution. Indeed, it has specifically been found that instead of requiring the addition of concentrated nitric acid or the like and thereby establishing a strong solution of nitric and chloric acids to be treated with nitrogen peroxide, eilicient reaction between the latter gas and a'solution simply prepared from water and sodium chlorate, calcium chlorate or the like (i. e. other salt of chloric acid of like solubility, the term chlorate or chlorate salt being used herein to designate such salts as distinguished from the acid itself) may be achieved under conditions which have apparently not been recognized or reached heretofore. Indeed we have found that mere attempts to carry `out the treatment of chlorate solution with nitrogen peroxide by following simple or conventional theories of gas and liquid contact reaction, are highly inadequate; whereas by observation of certain unusual operating criteria which have not previously been appreciated and which are embraced by the present invention a practical and workable process has now been achieved for these convenient materials.

In accordance with principles thus discovered, a` presently preferred mode of carrying out the invention involves passing a mixture of nitrogen peroxide and diluent gas, e. g. air, concurrently with a solution of a chlorate such as described below (both the NO2 concentration in the gas and the chlorate concentration in the solution having at least certain specific values) through a reaction zone where the solution is distributed in an extremely thin film over a surface area (provided by solution-supporting structure or other packing in the zone) of peculiarly large extent relative to the total volume of such zone. By such operation and by maintaining the proportion of nitrogen peroxide to chlorate within a convenient but significantly defined range, remarkably satisfactory results have been achieved, viz. a continuous and easily controlled output of chlorine dioxide, having suitable dilution and representing uniformly high and thus economic consumption of reagent materials (especially the chlorate). Such procedure, by reason also of its stability of control and its use of safe and inexpensive reactants, finds utility in practical manufacture of chlorine dioxide for many commercial purposes, including the service of flour treatment or the like.

'For better illustration of certain aspects of the process, the accompanying drawing shows schematically one example of an apparatus or tower useful in carrying out the invention, the depicted View being essentially a vertical section of such apparatus, set forth in the nature of a flow diagram with respect to supplied reagents and withdrawn products.

As indicated hereinabove, certain minimal relations between the product chlorine dioxide and the supplied reagent are critically desirable 4 for economic value in the process, for example in that at least 30% and preferably about 50% of the supplied nitrogen peroxide should be usefully converted by the desired reaction, and at least about 40% and preferably 50% or more of the chlorate should be likewise consumed. With such results the present process is of denite advantagain comparison with other methodssuch as those involving reaction between chlorine and sodium chlorite-that require a much more costly reagent than the chlorates.

Whereas under circumstances outside the limits expressed below the percent conversion of both chlorate and nitrogen peroxide to chlorine dioxide are apt to depend markedly both on the relative proportions of these materials as well as on their rates and conditions of feed under continuous operation, the present process is of special and unusual advantage in that so long as the described conditions are maintained, the conversion to chlorine dioxide based on the nitrogen peroxide input can be maintained constant over a wide range of total chlorine dioxide output. In consequence, for controlling the dose of chlorine dioxide being applied to any particular use, the operator of the present process can count on a specic proportion of chlorine dioxide for every quantity of nitrogen peroxide supplied, e. g. under preferred conditions approximately l volume of C102 for every 2 `volumes of NO2 introduced. The practical value of this feature, by way of ease of control as explained above, will now be readily appreciated.

For practice of the process in a manner now preferred, apparatus .of the general sort shown in the drawing has been found especially advantageous. The reaction tower I0 consists essentially of a long, upright, hollow cylinder constructed of glass, suitable plastic or other material inert to the reagents and reaction products involved. The cylinder may have a partition or false bottom I I, perforated to permit spent liquid to flow down through a lower chamber I2 and out through a discharge or waste pipe I3, which can have its lower end sealed in a body of such solution in a discharge vessel I4. Over the partition II the tower is packed, as more particularly described below, with granules, pellets, or the like, of suitable, inert material I5, e. g. to a height H above the partition. The region bounded vby the cylindrical wall of the tower and the horizontal planes at the partition II and the upper level I of the packed material may be defined as the reaction zone.

At the head of the tower, provision is made for introducing reagent solution, e. g. at a more or less continuous, controlled rate, as by a pipe I8 through which solution flows or trickles into a feed chamber I9. From the bottom of the latter, the solution then travels through a trap 20 and discharges upon the head of the packed column of material I5, so as to flow down through the latter, preferably at aA slow, controlled rate, and spreading out laterally as well as vertically so that the solid particles are substantially completely covered, where exposed, with a thin film of the liquid. If desired, advance of the liquid through the pipe I8 may be effected or promoted by maintaining a small suction or vacuum in the upper chamber I9 by appropriate connection to a supplemental conduit 22.

' Gaseous reagent, e. g. a gaseous mixture of a minor proportion of nitrogen peroxide and a major proportion of diluent gas, is, introduced at the head of the column, as through the conduit Nif-Booth S'Serial No;

nitrates '2d/'openingfiritoitheisiiacelabdte tire iiiakiific'ffs@ eration `v or iother A#mention here; {such compound atiparer'itlylso 'iuristable as to 5be relatively noni existeraitA i. 'e.li`n tli'at at ordinary temperatures anyfsuppo'sed N203 iivould in fact be or atleast According sito present understanding, the fohlo I nate should te one ihavii-ig rlati'veiy high schifbiiity `in vw-atei.- ysodi-uni rohl'orate :has been serio- `ioioi'fed -with 'excellent results, .and among other alkali 'and 'alkaline -`ea-i'hiniettal v''alts, -lll-thium, calcium and-magnesium chlorat'esare Very suseful. -Inde'ed calcium 'chloratec-having a very-:high solubility, appears substantially superior "even "-to sodium chlorate, while potassium chlorate, Vhavaingl la flow solubility of not more 'than about y0.6 molar is practically useless fin-:the present process.V Withllre'spectwboth@to suitability Vof chlorate'sand concentrations to be fe'mployed 'ith-'e process, Lthe following i'tabler'epres'ents test Tre# i sults obtained *at Varioslevels of /ohlorate con;-

In vthese `tests the lowest concentration represented use fof potassium c hlorate, the highest employed calcium ohloratepand the intermediate values from to v5.5 -mols'per liter were `solutions o'f "sodium `"chlorate It Will'be understood that all references herein to vinolarvalues of concentrationof Variouschloates are with respect toohlorate ionfife. mols 15er rliter-lof` C103.

Concentrations of less than .i2 molar are of -very littlev valuein `operatiin'is such as herein de scribed, the yield being'then less than 20%. A

elfiloratecontent"ofV not'les'sthanf3` inols per liter v is ofat least sorne utility, but peculiarly satisfactory fan'dthus distinctive results are achieved Within the Y presently 'preferred range. viz. -coh- @entretiens 'o'i about -5 molar or Higher, In 'the case of calcium misurate-solutions approaching 10 `rnclar'- (in'c'hl ateion) can be inad'e up and einployd'withf Vmeadvaritaec, 'although the iin'-l pr'ovem'ent 2in, yield rises fat fa ipingresen/ely 'Smaller rate -for concentrations firiuoh above `5 molar. Under bre'sently preferred conditions employing :sodium f'chl'ora'te at concentrations of 5 IITGIS 513er liter` or Slightly "-more. the yield v based oii'the chlorate'is about 50% to 60%. 11n the case of 'calcium ychloilata the f yield "basedv von ichlorate is' in the #neighborhood of 60% `for the use 0i cheiicallyequivalent c'lu'a'ntities 'of N92-fand caloifurn i'chio'fete, the v'yield :as based-fon No2 rising 'even-ifeoovoftoiy, 'tchathe-higher ioonoeiitiaiions of calcium chlorate. As an example of acon--` venient and. satisfactory reagent composition.

The volume concentration of nitrogen peroxidey in the suppliedvgaseous mixture, e. g. as introduced ,to the .tower through the pipe 24, is animportant factor. For results of substantial utility the concentration should be at least about 2% by volume (all references to gas concentrations herein being measured in volume percentage, unless otherwise stated); for example, even at a concentration of 2.5% in the supplied gas, the conversion of NO2 to chlorine dioxide is ordinarily well below the presently preferred value of 50% or so. At nitrogen peroxide concentrations above the yield comes quite close to 50%, optimum conditions being represented by an NO2 concentration of or more. For usual purposes the content of nitrogen peroxide should be no higher than about i. e. to avoid a chlorine dioxide concentration in the eiiiuent greater than a safe, non-explosive value of 10%. Present preference is to employ the nitrogen peroxide at a concentration of about 10%, the remaining 90% being suitable inert gas such as mentioned above. The diluent gas, and thus the gaseous mixture, may either be dry or contain some water r vapor. At the lower concentrations of NO2, it is of `advantage to humidify the diluting gas, because otherwise the large volume of the latter relative to the chlorate solution tends to evaporate sufficient water as to cause crystallization of the chlorate and consequent clogging of the reaction tower.

It is at present preferred to introduce the nitrogen dioxide in a proportion to the chlorate which is of the order of l mol of NO2 for each mol of chlorate ion supplied. Under such circumstances, e. g. operation at an intended or approximate control point where the NO2 and chlorate are present as chemical equivalents, a variation in the over-al1 ratio of the two reagents of plus or minus or so does not appear to alter the percent yield based on nitrogen 13er-, oxide; indeed up to excess NO2 may be employed without decreasing such yield. At 50% or greater excess, tests indicate that the yield based on NO2 decreases, and indeed the conversion factor then becomes undesirable dependent on the amount of NO2 supplied. Departures of the NO2 to chlorate ratio below unity, even to very low values, do not appear to affect the percent conversion of one gas to the other, but the utilization of chlorate becomes less and less as the feed ratio decreases, especially upon departure of more than 25% below the preferred NO2 to C103 ratio of 1:1. Some advantage, particularly where fairly good conjoint control of the solution feed and gas feed can be had, is obtained4 by em-V ploying up to, say 30% excess NO2 over the stoichiometric equivalents. In such case, the yield based on chlorate is markedly increased, e. g. to a value of 60% where the chlorate solution is 5.5 molar, andeven up to 70% when thechlorate concentration is as high as 7 .5 molar.

Within certain limits, the temperature is not highly critical for purposes of the present process. At very low temperatures in the reaction tower, however, the percent conversion of NO2 to chlorine dioxide is reduced; for instance at 13 C., .the ratio is affected to an extent ordinarily undesirable. Completely normal operation prevails at v25 C., and `even up to 40 C.

there little or no change in chlorine dioxide production. ..When temperatures of C. are reached a slight decrease in conversion is evidenced, although not of very significant extent. The reaction itself appears to generate some heat, so that in view of the above understanding of temperature conditions no attention need ordinarilybe paid to them providing the chlorate solution is not excessively cold when it enters the tower. In some cases, as where the apparatusrmay be situated in a cold locality and may be employed for extended periods at production rates far. below an Yintended average output, some thermal insulation around the reaction zone isdesirable to V.prevent adverse effects of low temp erature. l

In the light of many tests, 1t is found that the physical v or mechanical character of the reaction zone, and specifically the situation of the chlorate solution and its surface exposed to the reagent gas, are of extreme importance. Thus using apparatus of the sort indicated in the drawing. the tower should be packed in such a way as to produce a critically definite effect. More specifically, it appears that all of the solution flowing through the reaction zone should proceed as a thin film moving over the granules or particles or other distributed surface of the packing. If appreciable amountsof the solution are present in the tower in bulk, such as ordinary sized drops, the yield or conversion factor has been found to decrease materially. It also appears, perhaps as a corollary or a necessary structural prerequisite to the attainment of more immediately signicant conditions, that the spaces through which the gas moves should not be excessively large with respect to the particles or granules of packing over which the liquid runs. According to present experience, a factor of outstandingimportance may be defined as the surface area per unit volume, i. e. considering volume as representing the total volume of the reaction zone and including such volume as is occupied by the solidmaterial of the packing. Such ratio should apparently be high in itself, preferred results not having been obtained simply on provision of extended surface area without regard to other factors; a simple relation of output to surface area does not appear to exist in any such sense as that the quantitative output o f chlorine dioxide might be relied upon to increase or decrease (without affecting the yield) merely upon increasing or decreasing the area of exposed solution surface.

By way of illustration Vof a reaction zone arrangement found to be eminently suitable, a tower (of the general nature illustrated at I0) was employed, having an inside diameter of one inch, `.and filled with packing l5 to a height H of 69.5 cm. The granules of the packing were small alundum cylinders averaging 2.9 mm. in diameter by 2.88 mm. in length (i. e. altitude) and having an average weight of 0.0389 gram. The volume of the packed or reaction zone, e. g. between levels il and I6, was 352 cc. The alundum particles were packed down so that the quantity of them in such zone had a total weight of 431.5 grams. As determined simply from the gross dimensions, of the particles and without respect to their surface irregularity or porosity, whichv is nevertheless important as explained below, the total surface area of the packing was 0.436 square meter, with 40% voids, i. e. 40% of the volume of 352 cc. being empty space between granules-.availableforfpassage of liquid and gas.

VQperfetti.onffwith;ithisfowertMeleefrange0f rates of gaseous-reagent ed, reryeasatisfactery;

"Example, `I u 4The?phlorate; solutionaccnsstedfpf .water-fand '.xvn ainffed intmthe tower a heffrate.; 013312` liters :r-perr minute. r The :feed ratiotwasftnus 1 -'19,;2 mp1s whichrepresentedr50 :conversion ;of- Y een peroxide and 60% conversion of the sodium -zchlorata ,',Thusther product gaseous-mixture del:flivered -ffromxthe @1tower;.f -containeda; 2.5% ,C102 by ,f volume., ,1 mexgefuent-,spent liquid contained, fzln additionif-to -sodium ;-n1t1a te; i121* fgramsrper y itertofgzsodiumvcnlorate and -:12%==fbyi weighted .i ,f nitriof` acid.

Sodium-i chlorate solutionffof f;theY s a-mefnital oncentrationy as f im Example Iffwasffedffintthe 'fftower ati-the'ratezczrccwper minute. The-:slipplied gas mixture consisted`- off20% nitrogen per- `:oxide by volume in;A air, introduced at they rate `fof 1:35 liters'per minute,-thefreagent ratio being .r1.03imols;of'NOaper mol of'NaClOa. The; re-

sultingfchlorine dioxide production :was 23 grams viper hour, -representing@50% conversionofeach .1 of the reactants NO2I and sodium chlorate. mThe elluentfgas contained l10% 'C102 byzvolume; While #..the effluent s solution: contained f sodium; nitrate,

fand also 256, grams 'per literfofxsodium chlorate u and 18% Aby weight iofnitricfacid.

IAs further illustration`offeiective--reaction ,zone conditions,anotherl towers eused :had :an inside diameter of two inches andfwaslledfwith packing I5 to a height H of 291/4 inchesyeThe :pack- :sing again .f consistedof small alundum cylinders, averaging 3.5 7rnm.--inidiameter;and` 3.1 Vinfn,` in *fflengthfv Ieach Weighing, ony;the;v aaverage, vv0.0612 gram. fThe :totalifvoluntiey-oif;V the 1 reaction ,Zone,

granulare material being; packed down so;A 'that a AtotalJofl2036- .grams v'of:itii(i-. e.v thesmall Cylinders) lWas present in thezidened'- zone.l :The packng `4thusprovidedaetotal surface.area of 1.78 scuare fzltxempleiln @meras the "e: g. betweenl planes I I` andi I b21-Was'1590v cc.,l `the ,255,

-per literfofsodium chlorate -Wasfsupplied to the tower at a ratet10,',wapenminutetwhile -13.5 1iters-per-minute ofa aseousmigture containy fing 10 NO2; by,il olur,r1e--inV air-,Were concurrent- 5 1y introduced. ,Asin,-;al1otherexamplesherein fthe solutionj flowed or trickleddown `through the Ltower as a thinifflm. whichfappeaned t0 be distributed over substantially all, of the exposed alundum surfaces. {Ilhe `ratio ofsupplied reagents 10 was llmols of rNQz; perfmol of sodi-umchlorate.

The output a:easeous'fmixture-containing, slightv 1y less-than 5%ch1o1-,inedioxide comprised .108

gramsper hour. oi C lQz,v andrepresented 47% Con- :Versiim of the nitrogen -peroxdefand-.about -50 convetson.offsodium chlorate.

In thisrinstance'rof roperationwithf the twosinchf tower; chlora-te solution of rthe sameconcen- Y tration KWas*suppliedati210, cogner' rninuteg,J and :the :rreagentgas-mixture, consistngiofZQZ; nitrogen peroxi'defin airrfwas ,fintroducedsatfdf liters --per minute, affording ratio. Off 71.0'l-fmols of@ NOaper smol' of .chlorate "a-Theprodlmtonfwas ati thelate -ofz 211 grams; of'GlOrper hour;- p-roviding aconversion 0s- 44- f-of theme21-used. The output eas contained` about 9%; rchlorineldioxide. Since inriother:operationsitheametower `Was f runzatasflowL annoutput-as 14 grams of C102 per hour, withzNOz conyersionfof 441%;itmay be noted that with this: specificwarrangementgfofreaction zone,"the conversion factorffisg465- I-2;% overa production' ranger of 14rtQf2-1 is: 'gramsv of'fchlorine dioxideA per hour.

It -fisbelievedfthat, awiiigh` ratiofot -eiective re- 'aacting surface vf (oyerf Whichf-rthefliquidxspreads) lto vthe volumeffof ftheareactionfzone is; otrspecial .importance iin the:gaplfocesaqy e; gsffthe amount l of :reacting-surface'. per units.yolumel in-'the rtower, 40 f such volumef including" the espace1 occupied by the packing materiali as, lWell;4 asn the voidf space be- "ftween the.particlesfof-packing1 There-is also some-indicationrthati:theamount Dfffreactine Surfacefshouidabe :larger relativeatdrthefyod aSDaCe; "'itseems particularlylmdesirablerto ipermit the y ysolution` to collect'indriopso appreciable-magni- 1 v4tuale. f 'Furthermcrenthea useiful-forrfatual extent rof the reactina surece'zapnearstorba/determin- Iable'in largefrpa-r Fb ,materia-1 ofrifits-conx struction; ou rathergfbyfgthe,surface-congnration of..- sueh material-,1i inethat; a yen surtace ofrinovkTous'ialurrduing'forsexamplerbeingzfullyWettable ande-lawine a non-inolishedfonminutelyeporous characterfis ect much Irmre extended rthan f; -one lof; glass having fthe same grossfSdi-mensions.

Een examnleezns Oneetest the-etw@ :inch 'tower @described aboyefiwafflltdv; thvfthenSame-"fknd andisizefiot Smal-1 alu dum.vcYIindQYSsOmGWhat lessjtigntlyftpaeked,yizf1825 grams off-such.. maf-teria1-ecuprn 14701@ -aheieht-fH of -281/2 inches) fpf u 1clprovid-ing,f-aav otalftsurface., of 1.63 @square meters/tten f gross: dimensions i nithf .f vodspA Withfssueharr gement, Athe -conyerson gramsnem Qurmfwhleea ,constantficontersion f factorf otr r Li-(1% ,-tthreughoutf. a ,estimaient output 1- talige-1 represents' usefulcperatomsubstantial :e denarturefbelowsuchiyalueg-as Withloosennackso-called 30 11.9 and 12.3 respectively. These ratios, however, are based simply on the gross dimensions of the particles of packing. The effective surface area per unit volume was in an apparently important sense greater in such cases, because of the nature of the porous alundum used for the packing, than the numerical values indicate; where the towers were packed with glass particles of the same dimensions as the alundum particles in Examples I to IV respectively, the percent conversion of NO2 became much less satisfactory. The fact that with the glass particles or the more loosely packed alundum a higher conversion factor could not be achieved upon reducing the rate of introduction of reactants, indicates that the ratio of surface area to volume, rather than the magnitude of surface area alone, is the more important factor.

The foregoing criteria of the reaction zone surface have been confirmed by numerous other tests. Thus it has been found that other materials may be successfully used for the particles of packing (so long as they are appropriately inert in a chemical sense), e. g. pumice, silica geL'and other grades of aluminum oxide, all being substances having porosity or at least a porous or minutely rough surface.A Whereas in the examples specifically described above the alundum had a relatively high porosity, it was found that useful results (e. g. 43% conversion of NO2) were obtainable with alundum of only porosity. The advantage of porosity appears (as implied above) to reside in the greater surface area; alundum particles or Ygranules specially designed to have porosity only at or near the surface rather than internally, functioned as well as those made of material porous all the way through. Indeed as compared with glass (having an inherently polished and non-porous surface), even alundum'rated at zero porosity (when made in particles and packed to afford a surface-to-volume ratio of not less than about l2 as measured above) seemed to have suicient surface roughness to afford a minimum degree of utility, e. g. NO2 conversion of 34%, while garnet particles (non-porous, though minutely less smooth than glass) yielded a conversion factor of about 40%.

In the case of glass particles of ordinarily available character, even these minimum results were not obtained, although the particle size was decreased (and surface area likewise increased) to the limit where the iineness of subdivision made the packed mass so dense as to impede the passage of liquid and gas objectionably. Generally speaking, with particles of the preferred surface character, the most useful size range has been found to be from about 6 to 20 mesh, the above examples representing 6 to 8 mesh. With granules larger than 6 mesh, even of highly porous alundum, pumice or the like, the yield (i. e. conversion) falls off markedly;

p reducing the size from 6 to 10 mesh or so into moderately porous alundum), the stated numerical values of area-to-volume ratio are nevertheless of significance, even though the areas are only apparent areas, based on the gross dimensions of the particles. As measured in square centimeters of gross-dimension surface per cubic centimeter of reaction zone, ratios of about 12 and upward seem necessary for optimum results; indeed there is a slight increase in conversion factor (i. e. yield, based on NO2) with increase of the ratio up to about 28, but beyond such value of the ratio no improvement has been noted. At ratios below 12, the yield falls 01T, for example giving only a 40% conversion, under the specific conditions stated above, in the cited instance Where the ratio was about l1. As explained, these ratios are only significant as related to (or for comparison, when understood to involve) particles of packing ma terials having a substantially rougher surface than ordinary glass, the area-increasing factor of such roughness (or surface porosity) being not itself included in the numerical value of the ratio.

While in some cases countercurrent flow of gas and solution through the apparatus may provide a process of limited utility (e. g. with NO2 conversion of considerably less than 50%), concurrent iiow, such as the drawing indicates, has been found peculiarly superior, affording much more satisfactory results. YThe reasons for this difference have not b'een fully ascertained, although it is believed that with countercurrent flow the onrush of gas tends to pile up the solution in the form of droplets so that the reaction in thin films is not possible.

Within the proper limits and conditions as nov.' explained, the present process affords a safe, economical, and easily and reliably controlled way of generating chlorine dioxide. While the output gas may include som'e unreacted nitrogen peroxide and some chlorine, these incidental ingredients are not objectionable for most uses of the product, e. g. in treating flour or other materials. The procedure, moreover, can be carried out with relatively simple equipment, and without the use of costly or dangerous chemicals; it yields a steady flow of chlorine dioxide gas at any desired concentration that is compatible with safety.

It will be appreciated that whereas in practical performance of the process, adjustment can and preferably should be made of the chlorate solution feed as Well as of the nitrogen peroxide supply to the tower, great convenience is served by the described operations wherein precise correlation of the two reagents is unnecessary and where adjustment of one alone, viz. the gas, can be relied upon for corresponding, faithful control of the desired product.

It is`to be understood that the'invention is not limited to the specific embodiments herein set forth but may be carried out in other ways Within its spirit.

We claim:Y 1

1. A Vmethod of producing chlorine dioxide, comprisingl passing a gaseous mixture consisting of nitrogen peroxide and diluent gas and containing at least about 5% of nitrogen peroxide into a reaction zone of predetermined gross volume while advancing into said zone an aqueous chlorate solution consisting of water and soluble chlorate salt in concentration of chlorate of at least about k5 mols per liter, said solution being distributed as a thin Vfilm in said zone over a falargefeifective surface A area fproviddcbyl distributed f'guidingf-structure Awhich distributively fills -said zone,saidsurface'area being at least about as extensive `per cubic centimeter of the-gross l `volume ofthe yzoneas is provided-by a packing,

`in atotal-zonefoff352 cubic centimeters, of431.5 grams vof porous alunduin cylinders averaging 2.9 mnu-in diameter,2.8 mm. in length and'0.0389 gram in weight, said solution being caused to travel through-said zone and being @thereby-exposedjto the lgaseous mixture for reaction to -producech-lorine dioxide, withdrawing fromsaid re- MAactiorrzone ajgaseousmixture -containing diluent Agasand the'produced-'chlorine dioxide, and separately withdrawingrfromsaid zone Ythe -spent liquid which originated as said aqueous chlorate of nitrogen. peroxide and diluent gas ,and containing at least"2% "of nitrogen peroxide into a reaction. Zonepf ,predetermined gross volume 'chlorate solution consisting of water and soluble chlorate salt in,-,a concentration of chlorate ci at least 3 mols per liter, said solution being distrib- -uted in said-zone as a thin film overV a large cffective surface which is provided vby Aguidingstructure "distributively.occupying the gross volumeof the zone and which has a gross area of many square centimeters,.percubic` centimeter of saidjgross volume, .theveffective surface .area v,fof saidguiding structure `-percubic ,centimeter `of distributively occupied v volume being `at least equal tothat provided byaftotal surface .of 1.6 squaremeters 'of Valunduin .granules. having a porosity of"% in a Zone having a volume or 147i) cubic centimeters, said solution being therev by exposed to the gaseous mixture for reaction therewith to produce chlorine dioxide, and withdrawing from said reaction zone a gaseous mixture containing diluent gas and the produced chlorine dioxide, while also withdrawing from said Zone the spent liquid which originated as the aqueous chlorate solution, the introduced solution and gaseous mixture being advanced concurrently through the zone, and the nitrogen peroxide and chlorate being introduced in a molecular ratio not greater than 1.5:l.

3. A method of producing chlorine dioxide, comprising passing a gaseous mixture consisting of nitrogen peroxide and diluent gas and containing at least about 2% of nitrogen peroxide into a reaction zone of predetermined gross voiume while advancing into said zone an aqueous chlorate solution consisting of water and soluble chlorate salt in concentration of chlorate of at least about 3 mols per liter, said solution being distributed as a thin nlm in said zone over a large eiective surface area provided by distributed guiding structure which distributively lls said zone, said surface area being at least about as extensive per cubic centimeter of the gross volume of the zone as is provided by a packing, in a total zone of 352 cubic centimeters, of 431.5 grams of alundum cylinders averaging 2.9 mm. in diameter, 2.8 mm. in length and 0.0389 gram in weight, said solution being caused to travel rtv-throughtsaidfzone -andbeing gtherebyfexposed to jethe: :gaseous f mixture? lfor reaction ato produce chlorine dioxide, 1i the :introduced solution and .-'gaseous f mixture being advanced, concurrently Mrseparately withdrawing; from: said. zone` thel spent liquid which.` originated asi said. aqueous chlorate r' solution.

4.1A :methodfof .producing 5 chlorine dioxide, comprising passing: axgaseous'smixture consisting fofnitrogen.^peroxideand diluentgas and convtainingffattleastf2%of 'nitrogen-.peroxide into a #reaction rzone I of :.:predetermined gross, volume while vadvancing`r into 'said' zone an I aqueous chlorate: s'oli'itiorrconsistingl vof ywater andsoluble chlorate rrsalt'rj'in a concentration `.of iichlorate of at least'3 mols per' liter,:s'aid solution being distributed vinisai'd izone as Aa 'thin film over a large :f gross '-volume,ffifor f exposure "of fthe solution to the :gaseous :mixture vand reaction' therewith to yproduce :chlorine-'fidloxide and withdrawing from '-frlsaid reaction'zonef a' 'gaseous mixture-'containing rfdiluentz'- gas-f landthe vproduced chlorinefdioxide.

whileiy alsowithdrawinglfrom :said zoneI the spent liquid-which"originatedf'as the: aqueous chlorate -'=solution, i'fhe introduced solution and f gaseous f `mixture being @advanced concurrently through -Fthe zone, theesurface'foverV which'ithe solution isfdistributed in thezone having minuterough- :nessv at'4 leastequivalent 4to f that of garnet tand providingr-an 'effectivey area of-s'olution distributionicorrespond-irigly greater than representedby 'f 5.I A method-of producing *chlorine 1 dioxide,

' 'comprising passing a, gaseous mixture consisting of' `nitrogen peroxidev and diluent gas Aand Acontaining at least about'5%"of nitrogen peroxide 1 into-4 a reaction zone `fof predetermined gross volume=whle advancing Tinto said zone anaqueous chlorate solution consisting of water and soluble chlorate salt in concentration of chlorate of at least about 5 mols per liter, said solution being distributed as a thin lm in said zone over a large surface which is provided by guiding structure distributively occupyingV the gross volume of the zone and which has a gross area of at least about 11.5 square centimeters per cubic centimeter of said gross volume, said surface of the guiding structure having minute roughness at least equivalent to that of garnet, and said solution being caused to travel through said zone and being conned therein, as said thin film, to void v spaces having a total volume equal to not more than about 40% of the aforesaid gross volume of the Zone, said aqueous solution being thereby exposed to the gaseous mixture for reaction to produce chlorine dioxide, withdrawing from said reaction zone a gaseous mixture containing diluent gas and the produced chlorine dioxide and separately withdrawing from said zone the spent liquid which originated as said aqueous chlorate solution, said introduced gaseous mixture and aqueous chlorate solution being advanced concurrently through said zone until withdrawal as said second-mentioned gaseous mixture and spent liquid respectively, and said nitrogen peroxide and chlorate being introduced to the reaction zone in a molecular ratio of NO2 to chlorate ion between 0.5:1 and 1.5:1.

6. The method of claim wherein the solution is distributed over a surface provided by said guiding structure in said zone and having a gross area of at least about 12 square centimeters per cubic centimeter of said gross volume, said surface having minute roughness at least equivalent to that of alundum which has a porosity substantially greater than 30%, and said surface thereby providing an effective area, of solution distribution correspondingly greater than represented by said ratio of 12 square centimeters per cubic centimeter of gross volume.

7. In procedure for producing chlorine dioxide, the improvement which consists in passing nitrogen peroxide gas diluted with diluent gas amounting to a major proportion by volume of the total of said diluent and nitrogen peroxide, into a reaction zone of predetermined gross volume While advancing concurrently into said zone an aqueous solution consisting of water and soluble chlorate salt and having a chlorate concentration of at least 2 mols per liter, said solution as it passes through said zone being distributed as a thin'flm over guiding structure of solid material distributed throughout the zone, said solid material leaving only a, minor part of said gross volume as void spaces, and said guiding structure providing, in exposure to said void spaces, a large effective surface which distributes the solution in a lm at least as thin, and at least as extensive per cubic centimeter of said gross volume, as is provided by 6 to 8 mesh alundum particles packed to aiord 1l square centimeters of gross surface area per cubic centimeter of the space packed, said solution traveling through said zone concurrently with, and in exposure to. said dilute nitrogen peroxide, for reaction therewith to produce chlorine dioxide, and withdrawing from said reaction zone a gaseous mixture containing diluent gas and the produced chlorine dioxide, while also withdrawing from said zone the spent liquid which originated as the aqueous chlorate solution.

8. In procedure for producing chlorine dioxide, the improvement which consists in passing nitro- 16 gen peroxide gas diluted with diluent gas amounting to a major proportion by volume of the total of said diluent and nitrogen peroxide. into a reaction zone of predetermined gross volume while advancing concurrently into said zone an aqueous solution consisting of water and soluble chlorate salt and having a chlorate concentration of at least 3 mols per liter, said solution as it passes through said zone being distributed as a thin lxn over guiding structure of solid material distributed throughout the zone, said guiding structure providing a large, distributed, solution-receiving surface which has minute roughness at least equivalent to that of garnet and which has an area of many square centimeters per cubic centimeter of the aforesaid gross volume of the zone, said solution traveling through said zone as the aforesaid nlm and concurrently with, and in exposure to, said dilute nitrogen peroxide for reaction therewith to produce chlorine dioxide, and withdrawing from said reaction zone a gaseous mixture containing diluent gas and the produced chlorine dioxide, while also withdrawing from said zone the spent liquid which originated as the aqueous chlorate solution, said nitrogen peroxide and chlorate being introduced to the reaction zone in a molecular ratio of NO2 to chlorate ion not greater than 2:1.

HENRY C. MARKS. ROBERT R. JOINER.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,089,913 Cunningham Aug. l0, 1937 2,155,007 Edwards et al. Apr. 18, 1939 2,332,181 Soule Oct. 19, 1943 2,409,862 Hutchinson Oct. 22, 1946 2,451,826 Haller Oct. 19, 1948 2,475,286 Hutchinson July 5, 1949 

1. A METHOD OF PRODUCING CHLORINE DIOXIDE, COMPRISING PASSING A GASEOUS MIXTURE CONSISTING OF NITROGEN PEROXIDE AND DILUENT GAS AND CONTAINING AT LEAST ABOUT 5% OF NITROGEN PEROXIDE INTO A REACTION ZONE OF PREDETERMINED GROSS VOLUME WHILE ADVANCING INTO SAID ZONE AN AQUEOUS CHLORATE SOLUTION CONSISTING OF WATER AND SOLUBLE CHLORATE SALT IN CONCENTRATION OF CHLORATE OF AT LEAST ABOUT 5 MOLS PER LITER, SAID SOLUTION BEING DISTRIBUTED AS A THIN FILM IN SAID ZONE OVER A LARGE EFFECTIVE SURFACE AREA PROVIDED BY DISTRIBUTED GUIDING STRUCTURE WHICH DISTRIBUTIVELY FILLS SAID ZONE, SAID SURFACE AREA BEING AT LEAST ABOUT AS EXTENSIVE PER CUBIC CENTIMETER OF THE GROSS VOLUME OF THE ZONE AS IS PROVIDED BY A PACKING, IN A TOTAL ZONE OF 352 CUBIC CENTIMETERS, OF 431.5 GRAMS OF POROUS ALUNDUM CYLINDERS AVERAGING 2.9 MM. IN A DIAMETER, 2.8 MM. IN LENGTH AND 0.0389 GRAM IN WEIGHT, SAID SOLUTION BEING CAUSED TO TRAVEL THROUGH SAID ZONE AND BEING THEREBY EXPOSED TO THE GASEOUS MIXTURE FOR REACTION OR PRODUCE CHLORINE DIOXIDE, WITHDRAWING FROM SAID REACTION ZONE A GASEOUS MIXTURE CONTAINING DILUENT GAS AND THE PRODUCED CHLORINE DIOXIDE, AND SEPARATELY WITHDRAWING FROM SAID ZONE THE SPENT LIQUID WHICH ORIGINATED AS SAID AQUEOUS CHLORATE SOLUTION, SAID INTRODUCED GASEOUS MIXTURE AND AQUEOUS CHLORATE SOLUTION BEING ADVANCED CONCURRENTLY THROUGH SAID ZONE UNTIL WITHDRAWAL AS SAID SECOND-MENTIONED GASEOUS MIXTURE AND SPENT LIQUID RESPECTIVELY, AND SAID NITROGEN PEROXIDE AND CHLORATE BEING INTRODUCED TO THE REACTION ZONE IN A MOLECULAR RATIO OF NO2 TO CHLORATE ION BETWEEN 0.5:1 AND 1.5:1. 